Nonselective assembly of fibrillin 1 and fibrillin 2 in the rodent ocular zonule and in cultured cells: implications for Marfan syndrome

Secreted ADAMTS-like proteins as regulators of connective tissue function

The extracellular matrix (ECM) determines functional properties of connective tissues through structural components, such as collagens, elastic fibers, or proteoglycans. The ECM also instructs cell behavior through regulatory proteins, including proteases, growth factors, and matricellular proteins, which can be soluble or tethered to ECM scaffolds. The secreted a disintegrin and metalloproteinase with thrombospondin type 1 repeats/motifs-like (ADAMTSL) proteins constitute a family of regulatory ECM proteins that are related to ADAMTS proteases but lack their protease domains. In mammals, the ADAMTSL protein family comprises seven members, ADAMTSL1-6 and papilin. ADAMTSL orthologs are also present in the worm, Caenorhabditis elegans, and the fruit fly, Drosophila melanogaster. Like other matricellular proteins, ADAMTSL expression is characterized by tight spatiotemporal regulation during embryonic development and early postnatal growth and by cell type- and tissue-specific functional pleiotropy. Although largely quiescent during adult tissue homeostasis, reexpression of ADAMTSL proteins is frequently observed in the context of physiological and pathological tissue remodeling and during regeneration and repair after injury. The diverse functions of ADAMTSL proteins are further evident from disorders caused by mutations in individual ADAMTSL proteins, which can affect multiple organ systems. In addition, genome-wide association studies (GWAS) have linked single nucleotide polymorphisms (SNPs) in ADAMTSL genes to complex traits, such as lung function, asthma, height, body mass, fibrosis, or schizophrenia. In this review, we summarize the current knowledge about individual members of the ADAMTSL protein family and highlight recent mechanistic studies that began to elucidate their diverse functions.

Extracellular matrix protein composition dynamically changes during murine forelimb development

The extracellular matrix (ECM) is an integral part of multicellular organisms, connecting different cell layers and tissue types. During morphogenesis and growth, tissues undergo substantial reorganization. While it is intuitive that the ECM remodels in concert, little is known regarding how matrix composition and organization change during development. Here, we quantified ECM protein dynamics in the murine forelimb during appendicular musculoskeletal morphogenesis (embryonic days 11.5-14.5) using tissue fractionation, bioorthogonal non-canonical amino acid tagging, and mass spectrometry. Our analyses indicated that ECM protein (matrisome) composition in the embryonic forelimb changed as a function of development and growth, was distinct from other developing organs (brain), and was altered in a model of disease (osteogenesis imperfecta murine). Additionally, the tissue distribution for select matrisome was assessed via immunohistochemistry in the wild-type embryonic and postnatal musculoskeletal system. This resource will guide future research investigating the role of the matrisome during complex tissue development.

Marfan syndrome in a Ghanaian male: The diagnostic challenges

Marfan syndrome (MFS) is an autosomal dominantly inherited condition that has varying phenotypic expressions. This case report describes one such African patient, from Ghana, who had typical clinical and imaging traits of MFS but was first diagnosed incidentally at the age of 23 years. In this report, we explore the challenges of early diagnosis in this population.

Genetic mutation and aqueous humor metabolites alterations in a family with Marfan syndrome

This study used four generations of a Chinese family to reveal the genetic etiology and ocular manifestation pathogenesis of Marfan syndrome (MFS) through whole genome sequencing (WGS) and metabolomics analysis. In the study, we explored the pathogenic gene variant and aqueous humor (AH) metabolites alterations of MFS. Using WGS, a novel heterozygous variant (NM_000138: c.G4192A, p.D1398 N) in the fibrilin-1 (FBN1) gene was identified. This variant was co-segregated with the phenotype and considered "deleterious" and highly conserved during evolution. The p.D1398 N variant is located in a cbEGF-like domain and predicted to lead to a new splice site, which might result in structural and functional changes to the FBN1 protein. FBN1 is highly expressed in the mouse cornea, conjunctiva and lens capsule, which highlights the important role of FBN1 in eyeball development. AH metabolomics analysis identified eight differentially expressed metabolites, including 3-hydroxyphenylacetic acid, 4-pyridoxic acid, aminoadipic acid, azelaic acid, chlordiazepoxide, niacinamide, ribose, 1,5-bisphosphate and se-methylselenocysteine, associated with relevant metabolic pathways likely involved in the pathogenesis of ocular symptoms in MFS. Our analysis extends the existing spectrum of disease-causing mutations and reveals metabolites information related to the ophthalmic features of MFS. This may provide a new sight and a basis for the diagnosis and mechanism of MFS.

Genetic models of fibrillinopathies

The fibrillinopathies represent a group of diseases in which the 10-12 nm extracellular microfibrils are disrupted by genetic variants in one of the genes encoding fibrillin molecules, large glycoproteins of the extracellular matrix. The best-known fibrillinopathy is Marfan syndrome, an autosomal dominant condition affecting the cardiovascular, ocular, skeletal, and other systems, with a prevalence of around 1 in 3,000 across all ethnic groups. It is caused by variants of the FBN1 gene, encoding fibrillin-1, which interacts with elastin to provide strength and elasticity to connective tissues. A number of mouse models have been created in an attempt to replicate the human phenotype, although all have limitations. There are also natural bovine models and engineered models in pig and rabbit. Variants in FBN2 encoding fibrillin-2 cause congenital contractural arachnodactyly and mouse models for this condition have also been produced. In most animals, including birds, reptiles, and amphibians, there is a third fibrillin, fibrillin-3 (FBN3 gene) for which the creation of models has been difficult as the gene is degenerate and nonfunctional in mice and rats. Other eukaryotes such as the nematode C. elegans and zebrafish D. rerio have a gene with some homology to fibrillins and models have been used to discover more about the function of this family of proteins. This review looks at the phenotype, inheritance, and relevance of the various animal models for the different fibrillinopathies.

Proteolysis of fibrillin-2 microfibrils is essential for normal skeletal development

The embryonic extracellular matrix (ECM) undergoes transition to mature ECM as development progresses, yet few mechanisms ensuring ECM proteostasis during this period are known. Fibrillin microfibrils are macromolecular ECM complexes serving structural and regulatory roles. In mice, Fbn1 and Fbn2, encoding the major microfibrillar components, are strongly expressed during embryogenesis, but fibrillin-1 is the major component observed in adult tissue microfibrils. Here, analysis of Adamts6 and Adamts10 mutant mouse embryos, lacking these homologous secreted metalloproteases individually and in combination, along with in vitro analysis of microfibrils, measurement of ADAMTS6-fibrillin affinities and N-terminomics discovery of ADAMTS6-cleaved sites, identifies a proteostatic mechanism contributing to postnatal fibrillin-2 reduction and fibrillin-1 dominance. The lack of ADAMTS6, alone and in combination with ADAMTS10 led to excess fibrillin-2 in perichondrium, with impaired skeletal development defined by a drastic reduction of aggrecan and cartilage link protein, impaired BMP signaling in cartilage, and increased GDF5 sequestration in fibrillin-2-rich tissue. Although ADAMTS6 cleaves fibrillin-1 and fibrillin-2 as well as fibronectin, which provides the initial scaffold for microfibril assembly, primacy of the protease-substrate relationship between ADAMTS6 and fibrillin-2 was unequivocally established by reversal of the defects in Adamts6^-/- embryos by genetic reduction of Fbn2, but not Fbn1.

LTBP1 promotes fibrillin incorporation into the extracellular matrix

LTBP1 is a large extracellular matrix protein and an associated ligand of fibrillin-microfibrils. Knowledge of LTBP1 functions is largely limited to its role in targeting and sequestering TGFβ growth factors within the extracellular matrix, thereby regulating their bioavailability. However, the recent description of a wide spectrum of phenotypes in multiple tissues in patients harboring LTBP1 pathogenic variants suggests a multifaceted role of the protein in the homeostasis of connective tissues. To better understand the human pathology caused by LTBP1 deficiency it is important to investigate its functional role in extracellular matrix formation. In this study, we show that LTBP1 coordinates the incorporation of fibrillin-1 and -2 into the extracellular matrix in vitro. We also demonstrate that this function is differentially exerted by the two isoforms, the short and long forms of LTBP1. Thereby our findings uncover a novel TGFβ-independent LTBP1 function potentially contributing to the development of connective tissue disorders.

The fibrillinopathies: new insights with focus on the paradigm of opposing phenotypes for both FBN1 and FBN2

Different pathogenic variants in the fibrillin‐1 gene (FBN1) cause Marfan syndrome and acromelic dysplasias. Whereas the musculoskeletal features of Marfan syndrome involve tall stature, arachnodactyly, joint hypermobility, and muscle hypoplasia, acromelic dysplasia patients present with short stature, brachydactyly, stiff joints, and hypermuscularity. Similarly, pathogenic variants in the fibrillin‐2 gene (FBN2) cause either a Marfanoid congenital contractural arachnodactyly or a FBN2‐related acromelic dysplasia that most prominently presents with brachydactyly. The phenotypic and molecular resemblances between both the FBN1 and FBN2‐related disorders suggest that reciprocal pathomechanistic lessons can be learned. In this review, we provide an updated overview and comparison of the phenotypic and mutational spectra of both the “tall” and “short” fibrillinopathies. The future parallel functional study of both FBN1/2‐related disorders will reveal new insights into how pathogenic fibrillin variants differently affect the fibrillin microfibril network and/or growth factor homeostasis in clinically opposite syndromes. This knowledge may eventually be translated into new therapeutic approaches by targeting or modulating the fibrillin microfibril network and/or the signaling pathways under its control.

Association between anti-fibrillin-2 protein induced retinal degeneration via intravitreous delivery and activated TGF-β signaling in mice

Fibrillin-2 (FBN2) is a major component of tissue microfibrils, and the decrease of FBN2 perturbs the signaling events mediated by TGF-β, thereby playing a role in macular degeneration. However, the association between the retinal degeneration resulting from the abnormality of FBN2 and the activation of TGF-β signaling has not been fully addressed. In the present study, the mice were divided into a normal control group (NC group), a phosphate-buffered saline (PBS) injection group (PBS group), and an anti-FBN2 protein injection group (anti-FBN2 group), and the mice in PBS and anti-FBN2 groups received the relevant treatment via the intravitreal injection once a week for three consecutive weeks. One week later after injection, the retinal morphology and visual function of the fundus were detected. Further, the expression of FBN2, TGF-β1, TGF-β2 and TGF-β3 in retina was measured using quantitative polymerase chain reaction (Q-PCR) and enzyme-linked immunosorbent assay (ELISA), respectively. As a result, fundus examination suggests that after intravitreous injection of anti-FBN2 protein, there were a large patchy yellow white degeneration region and numerous pigmentations in the retina in anti-FBN2-treated mice; by contrast, there was no apparent change in mice from the NC and PBS groups. The retina suffered markedly damage, and the thickness of whole retina and outer nuclear layer markedly thinned. The expression of FBN2 was decreased whereas the levels of TGF-β1, TGF-β2, and TGF-β3 were upregulated. Together, our findings indicate that the intravitreous delivery of anti-FBN2 protein could induce retina degeneration in mice, accompanied by the higher activated TGF-β. The retinal degeneration mouse model established will provide a platform for the investigation of the retinal diseases.

Hyperkyphosis is not dependent on bone mass and quality in the mouse model of Marfan syndrome

Marfan syndrome (MFS) is an autosomal dominant disease affecting cardiovascular, ocular and skeletal systems. It is caused by mutations in the fibrillin-1 (FBN1) gene, leading to structural defects of connective tissue and increased activation of TGF-β. Angiotensin II (ang-II) is involved in TGF-β activity and in bone mass regulation. Inhibition of TGF-β signaling by blockage of the ang-II receptor 1 (AT1R) via losartan administration leads to improvement of cardiovascular and pulmonary phenotypes, but has no effect on skeletal phenotype in the haploinsufficient mouse model of MFS mg^R, suggesting a distinct mechanism of pathogenesis in the skeletal system. Here we characterized the skeletal phenotypes of the dominant-negative model for MFS mgΔ^lpn and tested the effect of inhibition of ang-II signaling in improving those phenotypes. As previously shown, heterozygous mice present hyperkyphosis, however we now show that only males also present osteopenia. Inhibition of ang-II production by ramipril minimized the kyphotic deformity, but had no effect on bone microstructure in male mutant animals. Histological analysis revealed increased thickness of the anterior longitudinal ligament (ALL) of the spine in mutant animals (25.8 ± 6.3 vs. 29.7 ± 7.7 μm), coupled with a reduction in type I (164.1 ± 8.7 vs. 139.0 ± 4.4) and increase in type III (86.5 ± 10.2 vs. 140.4 ± 5.6) collagen in the extracellular matrix of this ligament. In addition, we identified in the MFS mice alterations in the erector spinae muscles which presented thinner muscle fibers (1035.0 ± 420.6 vs. 655.6 ± 239.5 μm²) surrounded by increased area of connective tissue (58.17 ± 6.52 vs. 105.0 ± 44.54 μm²). Interestingly, these phenotypes were ameliorated by ramipril treatment. Our results reveal a sex-dependency of bone phenotype in MFS, where females do not present alterations in bone microstructure. More importantly, they indicate that hyperkyphosis is not a result of osteopenia in the MFS mouse model, and suggest that incompetent spine ligaments and muscles are responsible for the development of that phenotype.

Marfan syndrome

Marfan syndrome (MFS) is an autosomal dominant, age-related but highly penetrant condition with substantial intrafamilial and interfamilial variability. MFS is caused by pathogenetic variants in FBN1, which encodes fibrillin-1, a major structural component of the extracellular matrix that provides support to connective tissues, particularly in arteries, the pericondrium and structures in the eye. Up to 25% of individuals with MFS have de novo variants. The most prominent manifestations of MFS are asymptomatic aortic root aneurysms, aortic dissections, dislocation of the ocular lens (ectopia lentis) and skeletal abnormalities that are characterized by overgrowth of the long bones. MFS is diagnosed based on the Ghent II nosology; genetic testing confirming the presence of a FBN1 pathogenetic variant is not always required for diagnosis but can help distinguish MFS from other heritable thoracic aortic disease syndromes that can present with skeletal features similar to those in MFS. Untreated aortic root aneurysms can progress to life-threatening acute aortic dissections. Management of MFS requires medical therapy to slow the rate of growth of aneurysms and decrease the risk of dissection. Routine surveillance with imaging techniques such as transthoracic echocardiography, CT or MRI is necessary to monitor aneurysm growth and determine when to perform prophylactic repair surgery to prevent an acute aortic dissection.

Altered Ocular Fibrillin Microfibril Composition in Mice With a Glaucoma-Causing Mutation of Adamts10

Purpose

Previously, we identified a G661R mutation of ADAMTS10 (a disintegrin-like and metalloprotease with thrombospondin type 1 motif 10) as being disease causative in a colony of Beagles with inherited primary open-angle glaucoma (POAG). Mutations in ADAMTS10 are known to cause Weill-Marchesani syndrome (WMS), which is also caused by mutations in the fibrillin-1 gene (FBN1), suggesting functional linkage between ADAMTS10 and fibrillin-1, the principal component of microfibrils. Here, we established a mouse line with the G661R mutation of Adamts10 (Adamts10G661R/G661R) to determine if they develop features of WMS and alterations of ocular fibrillin microfibrils.

Methods

Intraocular pressure (IOP) was measured using a TonoLab rebound tonometer. Central cornea thickness (CCT), anterior chamber depth (ACD) and axial length (AL) of the eye were examined by spectral-domain optical coherence tomography. Sagittal eye sections from mice at postnatal day 10 (P10) and at 3 and 24 months of age were stained with antibodies against fibrillin-1, fibrillin-2, and ADAMTS10.

Results

IOP was not elevated in Adamts10G661R/G661R mice. Adamts10G661R/G661R mice had smaller bodies, thicker CCT, and shallower ACD compared to wild-type mice but normal AL. Adamts10G661R/G661R mice displayed persistent fibrillin-2 and enhanced fibrillin-1 immunofluorescence in the lens zonules and in the hyaloid vasculature and its remnants in the vitreous.

Conclusions

Adamts10G661R/G661R mice recapitulate the short stature and ocular phenotypes of WMS. The altered fibrillin-1 and fibrillin-2 immunoactivity in Adamts10G661R/G661R mice suggests that the G661R mutation of Adamts10 perturbs regulation of the fibrillin isotype composition of microfibrils in the mouse eye.

Nanophthalmos-Associated MYRF Gene Mutation Causes Ciliary Zonule Defects in Mice

Purpose

Patients with nanophthalmos who undergo intraocular surgery often present with abnormal ciliary zonules. In a previous study, we reported mutation in MYRF that is implicated in the pathogenesis of nanophthalmos. The aim of this study was to model the mutation in mice to explore the role of MYRF on zonule structure and its major molecular composition, including FBN1 and FBN2.

Methods

Human MYRF nanophthalmos frameshift mutation was generated in mouse using the CRISPR-Cas9 system. PCR and Sanger sequencing were used for genotype analysis of the mice model. Anterior chamber depth (ACD) was measured using hematoxylin and eosin–stained histology samples. Morphologic analysis of ciliary zonules was carried out using silver staining and immunofluorescence. Transcript and protein expression levels of MYRF, FBN1, and FBN2 in ciliary bodies were quantified using quantitative real-time PCR (qRT-PCR) and Western blot.

Results

A nanophthalmos frameshift mutation (c.789delC, p.N264fs) of MYRF in mice showed ocular phenotypes similar to those reported in patients with nanophthalmos. ACD was reduced in MYRF mutant mice (MYRF^mut/+) compared with that in littermate control mice (MYRF^+/+). In addition, the morphology of ciliary zonules showed reduced zonular fiber density and detectable structural dehiscence of zonular fibers. Furthermore, qRT-PCR analysis and Western blot showed a significant decrease in mRNA expression levels of MYRF, FBN1, and FBN2 in MYRF^mut/+ mice.

Conclusions

Changes in the structure and major molecular composition of ciliary zonules accompanied with shallowing anterior chamber were detected in MYRF^mut/+ mice. Therefore, MYRF mutant mice strain is a useful model for exploring pathogenesis of zonulopathy, which is almost elusive for basic researches due to lack of appropriate animal models.

Dynamics of the lens basement membrane capsule and its interaction with connective tissue-like extracapsular matrix proteins

The lens, suspended in the middle of the eye by tendon-like ciliary zonule fibers and facing three different compartments of the eye, is enclosed in what has been described as the thickest basement membrane in the body. While the protein components of the capsule have been a subject of study for many years, the dynamics of capsule formation, and the region-specific relationship of its basement membrane components to one another as well as to other matrix molecules remains to be explored. Through high resolution confocal and super-resolution imaging of the lens capsule and 3D surface renderings of acquired z-stacks, our studies revealed that each of its basement membrane proteins, laminin, collagen IV, nidogen and perlecan, has unique structure, organization, and distribution specific both to the region of the lens that the capsule is located in and the position of the capsule within the eye. We provide evidence of basal membrane gradients across the depth of the capsule as well as the synthesis of distinct basement membrane lamella within the capsule. These distinctions are most prominent in the equatorial capsule zone where collagen IV and nidogen span the capsule depth, while laminin and perlecan are located in two separate lamellae located at the innermost and outermost capsule domains. We discovered that an extracapsular matrix compartment rich in the connective tissue-like matrix molecules fibronectin, tenascin-C, and fibrillin is integrated with the superficial surface of the lens capsule. Each matrix protein in this extracapsular zone also exhibits region-specific distribution with fibrils of fibrillin, the matrix protein that forms the backbone of the ciliary zonules, inserting within the laminin/perlecan lamella at the surface of the equatorial lens capsule.

The mgΔlpn mouse model for Marfan syndrome recapitulates the ocular phenotypes of the disease

PURPOSE

Fibrillin-1 and -2 are major components of tissue microfibrils that compose the ciliary zonule and cornea. While mutations in human fibrillin-1 lead to ectopia lentis, a major manifestation of Marfan syndrome (MFS), in mice fibrillin-2 can compensate for reduced/lack of fibrillin-1 and maintain the integrity of ocular structures. Here we examine the consequences of a heterozygous dominant-negative mutation in the Fbn1 gene in the ocular system of the mgΔ^lpn mouse model for MFS.

METHODS

Eyes from mgΔ^lpn and wild-type mice at 3 and 6 months of age were analyzed by histology. The ciliary zonule was analyzed by scanning electron microscopy (SEM) and immunofluorescence.

RESULTS

Mutant mice presented a significantly larger distance of the ciliary body to the lens at 3 and 6 months of age when compared to wild-type, and ectopia lentis. Immunofluorescence and SEM corroborated those findings in MFS mice, revealing a disorganized mesh of microfibrils on the floor of the ciliary body. Moreover, mutant mice also had a larger volume of the anterior chamber, possibly due to excess aqueous humor. Finally, losartan treatment had limited efficacy in improving ocular phenotypes.

CONCLUSIONS

In contrast with null or hypomorphic mutations, expression of a dominant-negative form of fibrillin-1 leads to disruption of microfibrils in the zonule of mice. This in turn causes lens dislocation and enlargement of the anterior chamber. Therefore, heterozygous mgΔ^lpn mice recapitulate the major ocular phenotypes of MFS and can be instrumental in understanding the development of the disease.

Latent-transforming growth factor beta-binding protein-2 (LTBP-2) is required for longevity but not for development of zonular fibers

Latent-transforming growth factor beta-binding protein 2 (LTBP-2) is a major component of arterial and lung tissue and of the ciliary zonule, the system of extracellular fibers that centers and suspends the lens in the eye. LTBP-2 has been implicated previously in the development of extracellular microfibrils, although its exact role remains unclear. Here, we analyzed the three-dimensional structure of the ciliary zonule in wild type mice and used a knockout model to test the contribution of LTBP-2 to zonule structure and mechanical properties. In wild types, zonular fibers had diameters of 0.5-1.0 micrometers, with an outer layer of fibrillin-1-rich microfibrils and a core of fibrillin-2-rich microfibrils. LTBP-2 was present in both layers. The absence of LTBP-2 did not affect the number of fibers, their diameters, nor their coaxial organization. However, by two months of age, LTBP-2-depleted fibers began to rupture, and by six months, a fully penetrant ectopia lentis phenotype was present, as confirmed by in vivo imaging. To determine whether the seemingly normal fibers of young mice were compromised mechanically, we compared zonule stress/strain relationships of wild type and LTBP-2-deficient mice and developed a quasi-linear viscoelastic engineering model to analyze the resulting data. In the absence of LTBP-2, the ultimate tensile strength of the zonule was reduced by about 50%, and the viscoelastic behavior of the fibers was altered significantly. We developed a harmonic oscillator model to calculate the forces generated during saccadic eye movement. Model simulations suggested that mutant fibers are prone to failure during rapid rotation of the eyeball. Together, these data indicate that LTBP-2 is necessary for the strength and longevity of zonular fibers, but not necessarily for their formation.

Zinn's zonule

The Zonule of Zinn, or ciliary zonule, is the elaborate system of extracellular fibers that centers the lens in the eye. In humans, the fibers transmit forces that flatten the lens during the process of disaccommodation, thereby bringing distant objects into focus. Zonular fibers are composed almost entirely of 10-12 nm-wide microfibrils, of which polymerized fibrillin is the most abundant component. The thickest fibers have a fascicular organization, where hundreds or thousands of microfibrils are gathered into micrometer-wide bundles. Many such bundles are aggregated to form a fiber. Dozens of proteins comprise the zonule. Most are derived from cells of the non-pigmented ciliary epithelium in the pars plana region, although some are probably contributed by the lens and perhaps other tissues of the anterior segment. Zonular fibers are viscoelastic cables but their component microfibrils are rather stiff structures. Thus, the elastic properties of the fibers likely stem from lateral interactions between microfibrils. Rupture of zonular fibers and subsequent lens dislocation (ectopia lentis) can result from blunt force trauma or be a sequela of other eye diseases, notably exfoliation syndrome. Ectopia lentis is also a feature of syndromic conditions caused typically by mutations in microfibril-associated genes. The resulting ocular phenotypes raise the possibility that the zonule regulates lens size and shape, globe size, and even corneal topology, in addition to its well-recognized role in accommodation.

An immune response to the avascular lens following wounding of the cornea involves ciliary zonule fibrils

The lens and central cornea are avascular. It was assumed that the adult lens had no source of immune cells and that the basement membrane capsule surrounding the lens was a barrier to immune cell migration. Yet, microfibril‐associated protein‐1 (MAGP1)‐rich ciliary zonules that originate from the vasculature‐rich ciliary body and extend along the surface of the lens capsule, form a potential conduit for immune cells to the lens. In response to cornea debridement wounding, we find increased expression of MAGP1 throughout the central corneal stroma. The immune cells that populate this typically avascular region after wounding closely associate with this MAGP1‐rich matrix. These results suggest that MAGP1‐rich microfibrils support immune cell migration post‐injury. Using this cornea wound model, we investigated whether there is an immune response to the lens following cornea injury involving the lens‐associated MAGP1‐rich ciliary zonules. Our results provide the first evidence that following corneal wounding immune cells are activated to travel along zonule fibers that extend anteriorly along the equatorial surface of the lens, from where they migrate across the anterior lens capsule. These results demonstrate that lens‐associated ciliary zonules are directly involved in the lens immune response and suggest the ciliary body as a source of immune cells to the avascular lens.

Cell-Based Interaction Analysis of ADAMTS Proteases and ADAMTS-Like Proteins with Fibrillin Microfibrils

The extracellular matrix (ECM) is a composite biomaterial that serves as an anchor for cells and provides guidance cues for cell migration, proliferation, and differentiation. However, many details of the hierarchical ECM assembly process and the role of individual protein-protein interactions are not well understood. Here, I describe a cell-culture-based method that allows for determination of the ECM localization of recombinant ADAMTS proteases and ADAMTS-like (L) proteins in relationship to fibrillin microfibrils deposited by human dermal fibroblasts. The method can be readily adapted to study the localization of ECM components other than ADAMTS and ADAMTSL proteins to fibrillin microfibrils and other ECM networks.

A novel ADAMTS17 variant that causes Weill-Marchesani syndrome 4 alters fibrillin-1 and collagen type I deposition in the extracellular matrix

Weill-Marchesani syndrome (WMS) is a rare genetic disorder that affects the musculoskeletal system, the eye, and the cardiovascular system. Individuals with WMS present with short stature, joint contractures, thick skin, microspherophakia, small and dislocated lenses, and cardiac valve anomalies. WMS can be caused by recessive mutations in ADAMTS10 (WMS 1), ADAMTS17 (WMS 4), or LTBP2 (WMS 3), or by dominant mutations in fibrillin-1 (FBN1) (WMS 2); all genes encode secreted extracellular matrix (ECM) proteins. Individuals with WMS 4 due to ADAMTS17 mutations appear to have less severe cardiac involvement and present predominantly with the musculoskeletal and ocular features of WMS. ADAMTS17 is a member of the ADAMTS family of secreted proteases and directly binds to fibrillins. Here we report a novel pathogenic variant in ADAMTS17 that causes WMS 4 in an individual with short stature, brachydactyly, and small, spherical, and dislocated lenses. We provide biochemical and cell biological insights in the pathomechanisms of WMS 4, which also suggest potential biological functions for ADAMTS17. We show that the variant in ADAMTS17 prevents its secretion and we found intracellular accumulation of fibrillin-1 and collagen type I in patient-derived skin fibroblasts. In accordance, transmission electron microscopy revealed elastic fiber abnormalities, decreased collagen fibril diameters, and intracellular collagen accumulation in the dermis of the proband. Together, the data indicate a possible role for ADAMTS17 in the secretion of fibrillin-1 and collagen type I or in their early assembly in the pericellular matrix or the ECM.

A disintegrin-like and metalloproteinase domain with thrombospondin type 1 motif 9 (ADAMTS9) regulates fibronectin fibrillogenesis and turnover

The secreted metalloprotease ADAMTS9 has dual roles in extracellular matrix (ECM) turnover and biogenesis of the primary cilium during mouse embryogenesis. Its gene locus is associated with several human traits and disorders, but ADAMTS9 has few known interacting partners or confirmed substrates. Here, using a yeast two-hybrid screen for proteins interacting with its C-terminal Gon1 domain, we identified three putative ADAMTS9-binding regions in the ECM glycoprotein fibronectin. Using solid-phase binding assays and surface plasmon resonance experiments with purified proteins, we demonstrate that ADAMTS9 and fibronectin interact. ADAMTS9 constructs, including those lacking Gon1, co-localized with fibronectin fibrils formed by cultured fibroblasts lacking fibrillin-1, which co-localizes with fibronectin and binds several ADAMTSs. We observed no fibrillar ADAMTS9 staining after blockade of fibroblast fibronectin fibrillogenesis with a peptide based on the functional upstream domain of a Staphylococcus aureus adhesin. These findings indicate that ADAMTS9 binds fibronectin dimers and fibrils directly through multiple sites in both molecules. Proteolytically active ADAMTS9, but not a catalytically inactive variant, disrupted fibronectin fibril networks formed by fibroblasts in vitro, and ADAMTS9-deficient RPE1 cells assembled a robust fibronectin fibril network, unlike WT cells. Targeted LC-MS analysis of fibronectin digested by ADAMTS9-expressing cells identified a semitryptic peptide arising from cleavage at Gly2196-Leu2197 We noted that this scissile bond is in the linker between fibronectin modules III17 and I10, a region targeted also by other proteases. These findings, along with stronger fibronectin staining previously observed in Adamts9 mutant embryos, suggest that ADAMTS9 contributes to fibronectin turnover during ECM remodeling.

Limb- and tendon-specific Adamtsl2 deletion identifies a role for ADAMTSL2 in tendon growth in a mouse model for geleophysic dysplasia

Geleophysic dysplasia is a rare, frequently lethal condition characterized by severe short stature with progressive joint contractures, cardiac, pulmonary, and skin anomalies. Geleophysic dysplasia results from dominant fibrillin-1 (FBN1) or recessive ADAMTSL2 mutations, suggesting a functional link between ADAMTSL2 and fibrillin microfibrils. Mice lacking ADAMTSL2 die at birth, which has precluded analysis of postnatal limb development and mechanisms underlying the skeletal anomalies of geleophysic dysplasia. Here, detailed expression analysis of Adamtsl2 using an intragenic lacZ reporter shows strong Adamtsl2 expression in limb tendons. Expression in developing and growing bones is present in regions that are destined to become articular cartilage but is absent in growth plate cartilage. Consistent with strong tendon expression, Adamtsl2 conditional deletion in limb mesenchyme using Prx1-Cre led to tendon anomalies, albeit with normal collagen fibrils, and distal limb shortening, providing a mouse model for geleophysic dysplasia. Unexpectedly, conditional Adamtsl2 deletion using Scx-Cre, a tendon-specific Cre-deleter strain, which does not delete in cartilage, also impaired skeletal growth. Recombinant ADAMTSL2 is shown here to colocalize with fibrillin microfibrils in vitro, and enhanced staining of fibrillin-1 microfibrils was observed in Prx1-Cre Adamtsl2 tendons. The findings show that ADAMTSL2 specifically regulates microfibril assembly in tendons and that proper microfibril composition in tendons is necessary for tendon growth. We speculate that reduced bone growth in geleophysic dysplasia may result from external tethering by short tendons rather than intrinsic growth plate anomalies. Taken together with previous work, we suggest that GD results from abnormal microfibril assembly in tissues, and that ADAMTSL2 may limit the assembly of fibrillin microfibrils.

Targeted deletion of fibrillin-1 in the mouse eye results in ectopia lentis and other ocular phenotypes associated with Marfan syndrome

Fibrillin is an evolutionarily ancient protein that lends elasticity and resiliency to a variety of tissues. In humans, mutations in fibrillin-1 cause Marfan and related syndromes, conditions in which the eye is often severely affected. To gain insights into the ocular sequelae of Marfan syndrome, we targeted Fbn1 in mouse lens or non-pigmented ciliary epithelium (NPCE). Conditional knockout of Fbn1 in NPCE, but not lens, profoundly affected the ciliary zonule, the system of fibrillin-rich fibers that centers the lens in the eye. The tensile strength of the fibrillin-depleted zonule was reduced substantially, due to a shift toward production of smaller caliber fibers. By 3 months, zonular fibers invariably ruptured and mice developed ectopia lentis, a hallmark of Marfan syndrome. At later stages, untethered lenses lost their polarity and developed cataracts, and the length and volume of mutant eyes increased. This model thus captures key aspects of Marfan-related syndromes, providing insights into the role of fibrillin-1 in eye development and disease.

Summary: Targeted knockout of Fbn1 in the ciliary epithelium of the mouse eye undermines the structural and biomechanical integrity of the ciliary zonule and results in an ectopia lentis phenotype.

Adamts10 inactivation in mice leads to persistence of ocular microfibrils subsequent to reduced fibrillin-2 cleavage

Mutations in the secreted metalloproteinase ADAMTS10 cause recessive Weill-Marchesani syndrome (WMS), comprising ectopia lentis, short stature, brachydactyly, thick skin and cardiac valve anomalies. Dominant WMS caused by FBN1 mutations is clinically similar and affects fibrillin-1 microfibrils, which are a major component of the ocular zonule. ADAMTS10 was previously shown to enhance fibrillin-1 assembly in vitro. Here, Adamts10 null mice were analyzed to determine the impact of ADAMTS10 deficiency on fibrillin microfibrils in vivo. An intragenic lacZ reporter identified widespread Adamts10 expression in the eye, musculoskeletal tissues, vasculature, skin and lung. Adamts10^-/- mice had reduced viability on the C57BL/6 background, and although surviving mice were slightly smaller and had stiff skin, they lacked brachydactyly and cardiovascular defects. Ectopia lentis was not observed in Adamts10^-/- mice, similar to Fbn1^-/- mice, most likely because the mouse zonule contains fibrillin-2 in addition to fibrillin-1. Unexpectedly, in contrast to wild-type eyes, Adamts10^-/- zonule fibers were thicker and immunostained strongly with fibrillin-2 antibodies into adulthood, whereas fibrillin-1 staining was reduced. Furthermore, fibrillin-2 staining of hyaloid vasculature remnants persisted post-natally in Adamts10^-/- eyes. ADAMTS10 was found to cleave fibrillin-2, providing an explanation for persistence of fibrillin-2 at these sites. Thus, analysis of Adamts10^-/- mice led to identification of fibrillin-2 as a novel ADAMTS10 substrate and defined a proteolytic mechanism for clearance of ocular fibrillin-2 at the end of the juvenile period.

Fell-Muir Lecture: Fibrillin microfibrils: structural tensometers of elastic tissues?

Fibrillin microfibrils are indispensable structural elements of connective tissues in multicellular organisms from early metazoans to humans. They have an extensible periodic beaded organization, and support dynamic tissues such as ciliary zonules that suspend the lens. In tissues that express elastin, including blood vessels, skin and lungs, microfibrils support elastin deposition and shape the functional architecture of elastic fibres. The vital contribution of microfibrils to tissue form and function is underscored by the heritable fibrillinopathies, especially Marfan syndrome with severe elastic, ocular and skeletal tissue defects. Research since the early 1990s has advanced our knowledge of biology of microfibrils, yet understanding of their mechanical and homeostatic contributions to tissues remains far from complete. This review is a personal reflection on key insights, and puts forward the conceptual hypothesis that microfibrils are structural 'tensometers' that direct cells to monitor and respond to altered tissue mechanics.

The Structural Role of Elastic Fibers in the Cornea Investigated Using a Mouse Model for Marfan Syndrome

Purpose

The presence of fibrillin-rich elastic fibers in the cornea has been overlooked in recent years. The aim of the current study was to elucidate their functional role using a mouse model for Marfan syndrome, defective in fibrillin-1, the major structural component of the microfibril bundles that constitute most of the elastic fibers.

Methods

Mouse corneas were obtained from animals with a heterozygous fibrillin-1 mutation (Fbn1+/-) and compared to wild type controls. Corneal thickness and radius of curvature were calculated using optical coherence tomography microscopy. Elastic microfibril bundles were quantified and visualized in three-dimensions using serial block face scanning electron microscopy. Transmission electron microscopy was used to analyze stromal ultrastructure and proteoglycan distribution. Center-to-center average interfibrillar spacing was determined using x-ray scattering.

Results

Fbn1+/- corneas were significantly thinner than wild types and displayed a higher radius of curvature. In the Fbn1+/- corneas, elastic microfibril bundles were significantly reduced in density and disorganized compared to wild-type controls, in addition to containing a higher average center-to-center collagen interfibrillar spacing in the center of the cornea. No other differences were detected in stromal ultrastructure or proteoglycan distribution between the two groups. Proteoglycan side chains appeared to colocalize with the microfibril bundles.

Conclusions

Elastic fibers have an important, multifunctional role in the cornea as highlighted by the differences observed between Fbn1+/- and wild type animals. We contend that the presence of normal quantities of structurally organized elastic fibers are required to maintain the correct geometry of the cornea, which is disrupted in Marfan syndrome.

Unusual life cycle and impact on microfibril assembly of ADAMTS17, a secreted metalloprotease mutated in genetic eye disease

Secreted metalloproteases have diverse roles in the formation, remodeling, and the destruction of extracellular matrix. Recessive mutations in the secreted metalloprotease ADAMTS17 cause ectopia lentis and short stature in humans with Weill-Marchesani-like syndrome and primary open angle glaucoma and ectopia lentis in dogs. Little is known about this protease or its connection to fibrillin microfibrils, whose major component, fibrillin-1, is genetically associated with ectopia lentis and alterations in height. Fibrillin microfibrils form the ocular zonule and are present in the drainage apparatus of the eye. We show that recombinant ADAMTS17 has unique characteristics and an unusual life cycle. It undergoes rapid autocatalytic processing in trans after its secretion from cells. Secretion of ADAMTS17 requires O-fucosylation and its autocatalytic activity does not depend on propeptide processing by furin. ADAMTS17 binds recombinant fibrillin-2 but not fibrillin-1 and does not cleave either. It colocalizes to fibrillin-1 containing microfibrils in cultured fibroblasts and suppresses fibrillin-2 (FBN2) incorporation in microfibrils, in part by transcriptional downregulation of Fbn2 mRNA expression. RNA in situ hybridization detected Adamts17 expression in specific structures in the eye, skeleton and other organs, where it may regulate the fibrillin isoform composition of microfibrils.

Axial Length of the Eyeball Is Important in Secondary Dislocation of the Intraocular Lens, Capsular Bag, and Capsular Tension Ring Complex

Purpose. To analyze the patients with secondary dislocation of CTR and IOL within 5 years from cataract surgery, to determine predisposing factors. Methods. 16 eyes of 15 patients aged 66.2 ± 6.7 (from 49 to 82) with CTR/IOL complex dislocation within 5 years from cataract surgery were compared with 26 patients aged 67.1 ± 7.2 (from 53 to 85), implanted with CTR during cataract surgery to manage zonule dehiscence and did not dislocate for at least 5 years, in respect of cause, axial length and IOL power, refraction, coexistent pathology, and trauma. Results. Axial length of the eyeball was 23.8 ± 1.3 (from 21 to 29) in the group of patients with CTR/IOL dislocation and 20.7 ± 1.2 (from 19 to 24) in patients with no dislocation present (p = 0.008). Crystalline lens dislocation was diagnosed before surgery in 13 of 16 patients with CTR/IOL complex dislocation as opposed to 7 of 26 eyes in the control group (p = 0.01). Pseudoexfoliation was present in 50% and 58% in both groups, respectively. Traumatic dislocation was present in 8 patients, none of them with CTR/IOL dislocation (p = 0.04). Conclusion. Longer axial length may contribute to the failure of the CTR to prevent in-the-bag IOL dislocation. Traumatic dislocation appears to be well fixed with the CTR.

Fibrillin-1, induced by Aurora-A but inhibited by BRCA2, promotes ovarian cancer metastasis

While Aurora-A (Aur A) provokes, BRCA2 restrains primary tumorigenesis, the roles of Aur A and BRCA2 in cancer metastasis remains unclear. Here, we show that the metastatic promoting markers SLUG, FBN1, and MMP2, 9, 13 are either stimulated or suppressed by Aur A or BRCA2, but the metastatic suppressors E-cadherin, β-catenin, and p53 are either inhibited or promoted by Aur A or BRCA2, leading to enhanced or reduced cell migration and invasion. Further study suggests that FBN1 inhibits E-cadherin and β-catenin, but stimulates MMP2, 9, 13. Depletion of SLUG abrogates FBN1 and MMP9, but increases E-cadherin, while p53 decreases both SLUG and FBN1. Animal assays demonstrate that FBN1 promotes both ovarian tumorigenesis and metastasis. Clinically, overexpression of BRCA2 or Aur A in ovarian cancer tissues predicts good or poor overall and disease free survivals. High expression of SLUG or FBN1 indicates poor overall survivals, whereas high expression of FBN1 but not of SLUG predicts poor disease free survival. No significant associations between p53 expression and patient survivals were found. Overall, FBN1, acts at the downstream of Aur A and BRCA2, promotes ovarian cancer metastasis through the p53 and SLUG-associated signaling, which may be useful for ovarian cancer diagnosis and treatment.

Disruption of murine Adamtsl4 results in zonular fiber detachment from the lens and in retinal pigment epithelium dedifferentiation

Human gene mutations have revealed that a significant number of ADAMTS (a disintegrin-like and metalloproteinase (reprolysin type) with thrombospondin type 1 motifs) proteins are necessary for normal ocular development and eye function. Mutations in human ADAMTSL4, encoding an ADAMTS-like protein which has been implicated in fibrillin microfibril biogenesis, cause ectopia lentis (EL) and EL et pupillae. Here, we report the first ADAMTSL4 mouse model, tvrm267, bearing a nonsense mutation in Adamtsl4. Homozygous Adamtsl4(tvrm267) mice recapitulate the EL phenotype observed in humans, and our analysis strongly suggests that ADAMTSL4 is required for stable anchorage of zonule fibers to the lens capsule. Unexpectedly, homozygous Adamtsl4(tvrm267) mice exhibit focal retinal pigment epithelium (RPE) defects primarily in the inferior eye. RPE dedifferentiation was indicated by reduced pigmentation, altered cellular morphology and a reduction in RPE-specific transcripts. Finally, as with a subset of patients with ADAMTSL4 mutations, increased axial length, relative to age-matched controls, was observed and was associated with the severity of the RPE phenotype. In summary, the Adamtsl4(tvrm267) model provides a valuable tool to further elucidate the molecular basis of zonule formation, the pathophysiology of EL and ADAMTSL4 function in the maintenance of the RPE.

ADAMTS proteins as modulators of microfibril formation and function

The ADAMTS (a disintegrin-like and metalloproteinase domain with thrombospondin-type 1 motifs) protein superfamily includes 19 secreted metalloproteases and 7 secreted ADAMTS-like (ADAMTSL) glycoproteins. The possibility of functional linkage between ADAMTS proteins and fibrillin microfibrils was first revealed by a human genetic consilience, in which mutations in ADAMTS10, ADAMTS17, ADAMTSL2 and ADAMTSL4 were found to phenocopy rare genetic disorders caused by mutations affecting fibrillin-1 (FBN1), the major microfibril component in adults. The manifestations of these ADAMTS gene disorders in humans and animals suggested that they participated in the structural and regulatory roles of microfibrils. Whereas two such disorders, Weill-Marchesani syndrome 1 and Weill-Marchesani-like syndrome involve proteases (ADAMTS10 and ADAMTS17, respectively), geleophysic dysplasia and isolated ectopia lentis in humans involve ADAMTSL2 and ADAMTSL4, respectively, which are not proteases. In addition to broadly similar dysmorphology, individuals affected by Weill-Marchesani syndrome 1, Weill-Marchesani-like syndrome or geleophysic dysplasia each show characteristic anomalies suggesting molecule-, tissue-, or context-specific functions for the respective ADAMTS proteins. Ectopia lentis occurs in each of these conditions except geleophysic dysplasia, and is due to a defect in the ciliary zonule, which is predominantly composed of FBN1 microfibrils. Together, this strongly suggests that ADAMTS proteins are involved either in microfibril assembly, stability, and anchorage, or the formation of function-specific supramolecular networks having microfibrils as their foundation. Here, the genetics and molecular biology of this subset of ADAMTS proteins is discussed from the perspective of how they might contribute to fully functional or function-specific microfibrils.

Adamtsl2 deletion results in bronchial fibrillin microfibril accumulation and bronchial epithelial dysplasia--a novel mouse model providing insights into geleophysic dysplasia

Mutations in the secreted glycoprotein ADAMTSL2 cause recessive geleophysic dysplasia (GD) in humans and Musladin–Lueke syndrome (MLS) in dogs. GD is a severe, often lethal, condition presenting with short stature, brachydactyly, stiff skin, joint contractures, tracheal-bronchial stenosis and cardiac valve anomalies, whereas MLS is non-lethal and characterized by short stature and severe skin fibrosis. Although most mutations in fibrillin-1 (FBN1) cause Marfan syndrome (MFS), a microfibril disorder leading to transforming growth factor-β (TGFβ) dysregulation, domain-specific FBN1 mutations result in dominant GD. ADAMTSL2 has been previously shown to bind FBN1 and latent TGFβ-binding protein-1 (LTBP1). Here, we investigated mice with targeted Adamtsl2 inactivation as a new model for GD (Adamtsl2^−/− mice). An intragenic lacZ reporter in these mice showed that ADAMTSL2 was produced exclusively by bronchial smooth muscle cells during embryonic lung development. Adamtsl2^−/− mice, which died at birth, had severe bronchial epithelial dysplasia with abnormal glycogen-rich inclusions in bronchial epithelium resembling the cellular anomalies described previously in GD. An increase in microfibrils in the bronchial wall was associated with increased FBN2 and microfibril-associated glycoprotein-1 (MAGP1) staining, whereas LTBP1 staining was increased in bronchial epithelium. ADAMTSL2 was shown to bind directly to FBN2 with an affinity comparable to FBN1. The observed extracellular matrix (ECM) alterations were associated with increased bronchial epithelial TGFβ signaling at 17.5 days of gestation; however, treatment with TGFβ-neutralizing antibody did not correct the epithelial dysplasia. These investigations reveal a new function of ADAMTSL2 in modulating microfibril formation, and a previously unsuspected association with FBN2. Our studies suggest that the bronchial epithelial dysplasia accompanying microfibril dysregulation in Adamtsl2^−/− mice cannot be reversed by TGFβ neutralization, and thus might be mediated by other mechanisms.

Summary: The extracellular protein ADAMTSL2 is a crucial regulator of microfibril composition in the extracellular matrix of bronchial smooth muscle cells and influences bronchial epithelial function.

Insights on ADAMTS proteases and ADAMTS-like proteins from mammalian genetics

The mammalian ADAMTS superfamily comprises 19 secreted metalloproteinases and 7 ADAMTS-like proteins, each the product of a distinct gene. Thus far, all appear to be relevant to extracellular matrix function or to cell-matrix interactions. Most ADAMTS functions first emerged from analysis of spontaneous human and animal mutations and genetically engineered animals. The clinical manifestations of Mendelian disorders resulting from mutations in ADAMTS2, ADAMTS10, ADAMTS13, ADAMTS17, ADAMTSL2 and ADAMTSL4 identified essential roles for each gene, but also suggested potential cooperative functions of ADAMTS proteins. These observations were extended by analysis of spontaneous animal mutations, such as in bovine ADAMTS2, canine ADAMTS10, ADAMTS17 and ADAMTSL2 and mouse ADAMTS20. These human and animal disorders are recessive and their manifestations appear to result from a loss-of-function mechanism. Genome-wide analyses have determined an association of some ADAMTS loci such as ADAMTS9 and ADAMTS7, with specific traits and acquired disorders. Analysis of genetically engineered rodent mutations, now achieved for over half the superfamily, has provided novel biological insights and animal models for the respective human genetic disorders and suggested potential candidate genes for related human phenotypes. Engineered mouse mutants have been interbred to generate combinatorial mutants, uncovering cooperative functions of ADAMTS proteins in morphogenesis. Specific genetic models have provided crucial insights on mechanisms of osteoarthritis (OA), a common adult-onset degenerative condition. Engineered mutants will facilitate interpretation of exome variants identified in isolated birth defects and rare genetic conditions, as well as in genome-wide screens for trait and disease associations. Mammalian forward and reverse genetics, together with genome-wide analysis, together constitute a powerful force for revealing the functions of ADAMTS proteins in physiological pathways and health disorders. Their continuing use, together with genome-editing technology and the ability to generate stem cells from mutants, presents numerous opportunities for advancing basic knowledge, human disease pathways and therapy.

Immunohistochemical expression of fibrillin-1 and fibrillin-2 during tooth development

BACKGROUND AND OBJECTIVE

Oxytalan fibers are categorized as a microfibril assembly without elastin deposition, and are unique components in the periodontal ligament (PDL). However, little is known about their formation during PDL development. To clarify the mechanisms of oxytalan fiber formation in developing PDL, we performed immunohistochemical analysis to detect the direct expression of fibrillin-1 and fibrillin-2, which are major components of microfibrils.

MATERIAL AND METHODS

Frozen sections of lower molars from mice at several stages of growth were prepared without chemical fixation and decalcification using the film transfer method. Immunostaining was performed with anti-fibrillin-1 and -2, and anticytokeratin antibodies.

RESULTS

Fibrillin-1 was not expressed in the dental follicle during the crown forming stage. At postneonatal day 9, fibrillin-1 expression started with meshwork appearance between the epithelial cells from Hertwig's epithelial root sheath at the root dentin surface. Fibirillin-2 was detected much earlier than fibrillin-1 expression. Fibrillin-2 was expressed with a liner appearance, running parallel to the root axis in PDL, and was partially co-expressed with cytokeratin 14 expression in Hertwig's epithelial root sheath. Furthermore, we detected both fibrillin-1 and fibrillin-2 expression in human PDL. Fibrillin-1 was detected in fibers with a vertically oriented root axis in PDL. Fibrillin-2 was widely expressed in PDL, including around the epithelial cell rests of Malassez. Fibrillin-1 and fibrillin-2 were clearly co-expressed in thick fiber structures in human PDL.

CONCLUSION

Our results suggest that both fibrillin-1 and fibrillin-2 expression is required to form thick oxytalan fibers in PDL. Based on the expression patterns for fibrillin-1 and fibrillin-2, they have different functions during tooth root and PDL development. Early expression of fibrillin-2 may regulate dental epithelial cell behavior during root and PDL development.

Human eye development is characterized by coordinated expression of fibrillin isoforms

PURPOSE

Mutations in human fibrillin-1 and -2, which are major constituents of tissue microfibrils, can affect multiple ocular components, including the ciliary zonule, lens, drainage apparatus, cornea, and retina. However, the expression pattern of the three human fibrillins and an integral microfibrillar component, MAGP1, during human eye development is not known.

METHODS

We analyzed sections from human eyes at gestational weeks (GWs) 6, 8, and 11 and at 1 and 3 years of age with antibodies specific for each human fibrillin isoform or MAGP1, using immunofluorescence microscopy.

RESULTS

During embryonic development, each fibrillin isoform was detected in vascular structures bridging the ciliary body and the developing lens, hyaloid vasculature, and retina. In addition, they were present in the developing corneal basement membranes and lens capsule. MAGP1 codistributed with the fibrillin isoforms. In contrast, the juvenile zonule was composed of fibrillin-1 microfibrils containing MAGP1, but fibrillin-2 was absent and fibrillin-3 was only sparsely detected.

CONCLUSIONS

Fibrillin-1, -2, and, unique to humans, fibrillin-3 are found in various ocular structures during human embryonic eye development, whereas fibrillin-1 dominates the postnatal zonule. We speculate that vasculature spanning the ciliary body and lens, which elaborates fibrillin-2 and -3, may provide an initial scaffold for fibrillin assembly and zonule formation.

Early fibrillin-1 assembly monitored through a modifiable recombinant cell approach

Fibrillin proteins constitute the backbone of extra-cellular macromolecular microfibrils. Mutations in fibrillins cause heritable connective tissue disorders, including Marfan syndrome, dominant Weill-Marchesani syndrome, and stiff skin syndrome. Fibronectin provides a critical scaffold for microfibril assembly in cell culture models. Full length recombinant fibrillin-1 was expressed by HEK 293 cells, which deposited the secreted protein in a punctate pattern on the cell surface. Cocultured fibroblasts consistently triggered assembly of recombinant fibrillin-1, which was dependent on a fibronectin network formed by the fibroblasts. Deposition of recombinant fibrillin-1 on fibronectin fibers occurred first in discrete packages that subsequently extended along fibronectin fibers. Mutant fibrillin-1 harboring either a cysteine 204 to serine mutation or a RGD to RGA mutation which prevents integrin binding, did not affect fibrillin-1 assembly. In conclusion, we developed a modifiable recombinant full-length fibrillin-1 assembly system that allows for rapid analysis of critical roles in fibrillin assembly and functionality. This system can be used to study the contributions of specific residues, domains, or regions of fibrillin-1 to the biogenesis and functionality of microfibrils. It provides also a method to evaluate disease-causing mutations, and to produce microfibril-containing matrices for tissue engineering applications, for example, in designing novel vascular grafts or stents.